The human gut ecosystem consists of a variety of different habitats and metabolic niches that are colonised by the so-called microbiota that contain more than 1011 micro-organisms per gram wet weight of contents, predominantly anaerobes (Macfarlane et al. 1997). The intestinal microbiota have both beneficial and pathogenic potential (Fuller & Gibson, 1997). The microbial community can provide protection against pathogenic bacteria, stimulates cell-mediated and humoral immune responses, and indirectly supports digestive processes by microbial fermentation (Berg, 1996; Cummings & Macfarlane, 1997). It also includes potential pathogenic organisms such as certain species of Clostridium, Escherichia, Salmonella, Shigella and Pseudomonas, as well as yeasts such as Candida albicans (Salminen et al. 1995). Evidence exists that the best protection against mucosal attachment and invasion by such pathogens is by keeping intestinal microbiota in a state that affords colonisation resistance against pathogens by modulation of the microbiota and by inducing luminal or systemic effects which are beneficial to the host's health. This may be achieved by the consumption of non-digestible food ingredients such as inulin-type fructans, known as prebiotics, which favour the growth and activity of certain colonic bacteria, such as bifidobacteria and lactobacilli, generally regarded as beneficial to the host.
However, not only the composition of the microbiota, but also the interaction of bacteria with the mucus layer and/or with the intestinal mucosa is important. The mucus layer is formed by high-molecular-weight mucins secreted by the goblet cells, secretory proteins and polymers mainly composed of polysaccharides. In addition, other glycoconjugates are also present; mainly glycoproteins and glycolipids synthesised by most of the epithelial cells producing the glycocalyx (Freitas & Cayuela, 2000). While these mucins and polymers may form a barrier against colonisation by some bacteria, other bacteria can use them as a means to adhere to the surface. Adherence leads to the formation of an adhesive microbial layer of one species that subsequently may support colonisation of other micro-organisms through co-adherence, promoting the development of microcolonies and biofilms.
It is accordingly, the combination of an intact intestinal mucosa covered with a biofilm of non-pathogenic bacteria that represents a barrier to the unrestrained uptake of antigens and pro-inflammatory molecules, including bacteria and bacterial products. When the normal microbiota, the mucus layer or the epithelial cells are disturbed by pathogens, antigens and other toxic substances from the gut lumen, defects in the barrier system become evident (Neish, 2002). Such a compromised mucosal barrier may increase paracellular permeability of the mucosa. As a consequence, the probability of an invasion by bacteria, antigens and pro-inflammatory molecules of the intestinal mucosa is increased under these conditions, resulting in inflammatory and immunologic responses (Lu & Walker, 2001). Because the first contact of bacteria with the intestinal tissue is with the mucus layer, which covers the underlying epithelium, consisting of enterocytes, columnar in shape, with an apical and a basolateral side and microvilli on the apical side, it represents an important element in the first line of defense against the invasion by pathogens, antigens or other harmful substances.
Therefore, the best protection against mucosal attachment and invasion by pathogens or other harmful substances could be the maintenance of the normal microbiota adhering tenaciously to mucus overlaying the mucosa. It is an object of the present invention to provide particular preparations useful in realizing the above, i.e. maintaining and/or improving the adherence of the normal microbiota to the mucosal layer.
In another aspect, more and more scientific data indicate that the intestinal microbiota impacts energy and metabolic homeostasis of the host, i.e. control of food and energy intake, food and energy metabolism, fat mass development, and associated metabolic disorders such as obesity and type 2 diabetes (Cani and Delzenne, 2007). Several mechanisms are now proposed that link events occurring in the colon and the regulation of energy metabolism. Intuitively, the major part of the microbiota is present at a point in the gut where food products escaped host digestion. By using these products, they intervene in host metabolism to provide energy through the production of metabolites absorbed by colonic host cells (short chain fatty acids). But they intervene also through more indirect ways. The following, non-limiting list of mechanisms may play an important role in metabolic homeostasis:                Propionate is a microbial metabolite which reduces cholesterol and triglyceride synthesis (Delzenne, 2007).        Impact of the gut microbiota on the fasting-induced adipose factor (FIAF) in the gut, which inhibits the activity of the enzyme lipoprotein lipase (LPL) (Backhed, 2004). This enzyme controls the release of fatty acids in the muscles and adipose tissue.        Type 2 diabetes and obesity are closely associated to a low-tone inflammatory state in response to being fed a high-fat diet. The bacterial lipopolysaccharide (LPS) from the Gram-negative intestinal microbiota may play an important role in this process, as absorbed lipopolysaccharide triggers the secretion of proinflammatory cytokines when it binds to the complex of CD14/TLR4 at the surface of immune cells (Cani and Delzenne, 2007).        Modulation of the production of gut peptides could constitute a link between bacterial fermentation in the lower part of the gut and systemic consequences of “colonic food” intake (Delzenne, 2007). More in particular such peptides either directly act as hormones modulating downstream metabolic processes, or indirectly trigger the production of hormones modulating such processes. In addition to others, glucagon-like peptide-1 and -2 (GLP-1 and GLP-2) are key hormones released in response to nutrient ingestion. They are produced by processing of their precursor proglucagon and promote insulin secretion (and sensitivity) and b-cell proliferation in the pancreas, control glycogen synthesis in muscle cells, and promote satiety. An increase in proglucagon mRNA and GLP-1 or -2 levels in the proximal colon are key events in the interaction between gut microbiota and metabolic homeostasis of the host.        
Arabinoxylan (AX), the main non-starch polysaccharide of cereal grains, is a dietary fiber constituent. These complex carbohydrates occur in cell walls of the starchy endosperm cells and the aleurone layer in most cereals (60-70% (w/w) of the total carbohydrate). They can be found in the endosperm cell walls of barley (20% (w/w)) and rice (40% (w/w)). Non-endospermic tissues of wheat, particularly the pericarp and testa, also contain a very high concentration of AX (64% (w/w)). AX consist of □-(1,4)-linked D-xylopyranosyl residues to which □-L-arabinofuranose units are linked as side chains (FIG. 1a). Some arabinoses can be substituted with ferulic acid. The degree of substitution refers to the arabinose moieties on the xylose backbone and is further also described as A/X ratio. The substitution and distribution of side chains are important factors in the physicochemical properties of AX. As for other polymers, also the degree of polymerization (DP), i.e. the molecular weight ratio of the polymer vis-à-vis the molecular weight of the repeating units, is an important factor in the physicochemical properties of AX. As used herein, the degree of polymerization is determined according to Courtin et al. (J. Chromatograph. A866 (2000) 97-104), i.e. measuring the number of reducing end xylose residues as repeating units.
AX are present in water-extractable form in grains and in a water-unextractable fraction present in the cell wall material. Whereas the latter one needs to be extracted from wheat using for instance alkali treatment, the water-extractable fraction is readily available in the watery waste streams from the wheat processing (Maes and Delcour 2002). In the current state of the art AX are extracted by using enzymes (i.e. hemicellulases and endoxylanases) which leads to (partial) hydrolysis of AX and results in a mixture of soluble and non-soluble AX molecules with low molecular weight (WO 199402874), (WO 2006027), (WO2006/002495), (U.S. Pat. No. 6,558,930). The reason for the focus on short chain—hydrolyzed—AX and the use of hemicellulases is that the yield of soluble long-chain AX is supposed to be too low. One reason for this low yield is the fact that endogenous enzymes are likely to degrade long-chain AX. Another reason is that the inherent viscosity of long-chain AX poses challenges for an efficient and cost effective extraction. It is also generally assumed that long-chain AX are badly water soluble because of their viscous nature. Finally, long-chain AX are not widely used for obtaining health/physiological effects, as it is currently assumed that only the short chain AX (soluble and non-soluble) have interesting physiological effects.
For all these reasons, the prior art describes the combined extraction of soluble and non-soluble AX with the aid of enzymes such as hemicellulases, endoxylanases etc resulting in short chain AX. Indeed, pre-biotic effects have so far only been described with AX of low molecular mass (see below).
There are also structural and functional differences that have been described between both soluble and non-soluble fractions (Glei, Hofmann et al. 2006). It is known from the literature (Garcia et al (2006) that long-chain AX have effects on glycaemic control in pre-diabetics, whilst such effects are not described for short chain AX.
The current invention surprisingly showed that: (i) methods are available that lead to acceptable yields of long chain soluble AX; (ii) hydrolysis or degradation of long-chain AX by the aforementioned enzymes can be largely avoided; (iii) long chain soluble AX have potent pre biotic and other systemic and non-systemic physiological effects, and (iv) the long chain AX preparations of the present invention have a good solubility profile.
Non digestible oligosaccharides (NDOs) like arabinoxylans, resist digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. These carbohydrates help to maintain regularity of colonic functions and could possibly contribute to human health by reducing the risk of chronic diseases. A lot of NDOs are considered to be prebiotics. Certain short chain arabinoxylans are also known to improve the growth of beneficial bacteria in the colon (WO2006002495 (A1), Grasten, 2003), (WO 2006/002495), however, no improved barrier function or selective growth and attachment of beneficial bacteria in the distal part of the colon were observed.
Prebiotics are non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of beneficial bacteria in the colon, thereby improving the host health (Gibson and Roberfroid 1995). Prebiotic effects in the gut can be evaluated on the basis of the growth of health promoting bacteria such as lactobacilli and bifidobacteria, the decrease in intestinal pathogens and the increase or decrease in production of health related bacterial metabolites. The latter include for instance short chain fatty acids, which are generally believed to be positive for colonic health, but also polyamines and ammonia, which are regarded as a risk factor for colon carcinogenesis.